They were stored to the simutrans graphics archieve before, which seems to be on the same server already. I was not active around the time that archive was actively used, so I don't know if it ever was widespread and well-known, or always operated under the radar.
Maybe we could tweak the attachment functionality of this forum a bit, so bigger attachments within the contest area are possible, allowing users to upload their screenshots instead of linking them. Or even add a gallery-feature to this forum... I don't know what's possibly.